The mayor’s decades-old control of the city budget and supplemental spending will come under challenge tonight.

The City Council is scheduled to take action on Mayor Stephanie Rawlings-Blake’s veto of two bills that the legislative body passed last month limiting the mayor’s power over expenditures.

If the vetoes are overridden – and that’s a big “if” because 12 of 15 Council votes are required – the measures would be placed on the November general ballot as charter amendments.

If that happens and voters approve the charter amendments, the running of City Hall would take on a different political calculus come December, when the next mayor and Council take office.

The first bill allows the Council to amend the annual budget to increase spending on existing programs or add funds for new purposes. Presently, the Council can cut spending, but cannot increase it, and “supplementary appropriations” can only be enacted by the Board of Estimates.

The second bill shrinks the Board of Estimates from five to three members. Presently, the mayor controls the panel – and day-in-and-day-out spending – by virtue of her vote and the votes of her two appointees, the city solicitor and director of public works.

Power Sharing

With the city solicitor and public works directors stripped of voting rights, the mayor would need the support of the City Council president or the city comptroller to pass spending items, resulting in de-facto power sharing.

Council President Bernard C. “Jack” Young, who has championed both bills, would be the biggest beneficiary of the change. He and five-term Comptroller Joan M. Pratt won last month’s Democratic primary and are expected to sail to victory in the November general election.

The current mayor, Rawlings-Blake, and the prospective mayor, state Sen. Catherine E. Pugh who has declared victory in the Democratic primary, are just as strenuously against the bills.

Given their opposition, it is uncertain whether either bill will pass the 12-vote hurdle tonight.

The Board of Estimates change has the narrowest path to an override.

The bill was enacted on a 12-1-2 vote, with Councilman Nick Mosby voting “no” and Mary Pat Clarke and Brandon M. Scott abstaining. If just one more affirmative vote is peeled off, the mayor’s veto will be sustained.

The Council spending measure has a somewhat better chance. The original vote was 14-1, with only Scott opposed.

But that was last month. Tonight the stakes and the behind-the-scenes tug of war over Council votes are much higher.